Assembly of thioether-containing rod-like liquid crystalline materials assisted by hydrogen-bonding terminal carboxyl groups

Abstract

We designed a hydrogen-bonding tolane-based molecule with terminal carboxyl and alkylsulfanyl groups in an effort to realize thioether-containing rod-like liquid crystalline materials. The molecule successfully exhibits a stable enantiotropic mesophase, in contrast to non-hydrogen bonding derivatives, including tolane and diphenyldiacetylene. In addition, it shows two remarkable characteristics compared to analogues with alkyl or alkoxy groups. First, the mesophase of the alkylsulfanyl derivative shows strong long-range correlation. Second, the birefringence of the alkylsulfanyl derivative is highly temperature-dependent, achieving a maximum value of 0.36. These remarkable characteristics are believed to be due to the high polarisability of sulfur atoms and S–S contacts. These findings will be helpful for the design of novel sulfur-containing rod-like liquid crystalline materials.

---

Introduction

The development of liquid crystalline (LC) materials containing sulfur atoms as mesogenic moieties has been actively pursued in recent years. These compounds are of interest because of their polarisabilities and van der Waals interactions (comprising dipole interactions due to high polarisabilities and the so-called S–S contacts derived from intra- or intermolecular n–σ* interactions between sulfur atoms), which are greater and stronger, respectively, than their more common analogues containing carbon and oxygen. The latter, in particular, are very important in organic electronics¹ as well as the molecular structural chemistry² of not only LCs but also crystalline materials, because they can lead to well-organized and/or specific molecular assemblies. Therefore, sulfur-containing LC materials including heterocyclic,³ disc-,⁴ and bent-type⁵ LC compounds with thioether (alkylsulfanyl) groups (–SR) have been widely reported.

However, rod-like (calamitic) LC compounds with alkylsulfanyl groups are quite uncommon. This is because even though sulfur atoms have beneficial effects on birefringence properties and dielectric constants, the rod-like compounds tend to crystallize (i.e. they show no mesophase) or exhibit an unstable monotropic mesophase owing to their strong van der Waals attractions.⁶ Cross et al. reported the butylsulfanyl group (–SC₄H₉) as a good candidate for the design of materials with high optical anisotropy; however, no information on stable mesophase was disclosed.⁶ To the best of our knowledge, there has been no detailed report on the mesophase structures and birefringence properties of rod-like LC materials with alkylsulfanyl groups.

Our recent interests have focused on the design of highly birefringent rod-like LCs and their application as optical materials.^3g,7–9 If a nematic phase could be obtained from a thioether-containing rod-like mesogen, an improvement in the birefringence properties of the optical material would be achieved.

To develop novel sulfur-containing rod-like LC materials, we focused on the utilization of hydrogen bonding. Molecules with carboxyl groups are known to generate mesophases owing to the formation of dimers in single-component systems,¹⁰ and since the discovery of hydrogen-bonding LC materials in multi-component systems by Kato et al.,¹¹ a large number of studies have been directed at their preparation, characterization, and practical application.^4a,12

Herein, we report the synthesis and characterization of a LC phase generated by hydrogen bonding in crystalline rod-like molecules containing an alkylsulfanyl group at the terminal position. Diphenylacetylene compounds with a hexyl (–C₆H₁₃; compound 1), hexyloxy (–OC₆H₁₃; compound 2), or hexylsulfanyl (–SC₆H₁₃; compound 3) chain at one terminus and a carboxyl group at the other were designed and prepared (Chart 1). These molecules, as so-called tolane-based LC materials, successfully formed well-defined enantiotropic mesophases, and their mesophase structures and birefringence (Δn) as an optical property were elucidated and measured, respectively. Interestingly, the alkylsulfanyl derivative exhibited highly correlated long-range mesophases and high birefringence compared with the alkyl and alkoxy derivatives.

Chart 1 Rod-like molecules in this study: tolanes with carboxylic acid (1–3), and other thioether-containing molecules (T1–T6 and DPDA-SC6).

Experimental

Measurements

The ¹H NMR and ¹³C NMR spectra were measured used a JEOL LNM-EX 400 at room temperature. CDCl₃ and DMSO-d₆ were used as measurement solvents with tetramethylsilane (TMS) as an internal standard. The liquid crystalline textures were investigated by polarizing optical microscopy (POM) (Leica DM2500P microscopy with a Mettler FP90 hot stage) and the transition temperatures and enthalpy changes were measured by differential scanning calorimetry (DSC) (Perkin Elmer DSC7) with heating and cooling scans performed at 10 °C min⁻¹. X-ray investigations were carried out with samples kept in glass capillary tubes (1.5 mm diameter) for oriented patterns under a magnetic field. WAXD measurements were conducted using a Bruker D8 DISCOVER equipped with a Vantec-500 detector using Cu-Kα radiation. The measurements of Δn were performed with a uniaxial aligned nematic cells. The transmittance of light under crossed Nicols conditions was observed as a function of wavelength by a microscope spectroscopic method using a Nikon LV100 Pol optical microscope equipped with a USB4000 (Ocean photonics) spectrometer.

Materials

1-Ethynyl-4-hexylbenzene, 4-iodophenol, trimethylsilylacetylene, 4-bromobenzoic acid, 4-bromobenzenethiol, ethynylbenzene, 4-bromobenzonitrile and Pd(PPh₃)₄ were purchased from TCI, and 6-bromohexane, triethylamine (TEA) and PPh₃ were purchased from Wako, and CuI was purchased from Kanto Chemical. Unless otherwise noted, all chemical were commercially available and use as received.

Synthetic procedure

1-Bromo-4-hexylsulfanylbenzene (3a). A mixture of 4-bromobenzenethiol (1.0 g, 5.3 mmol), 1-bromohexane (0.87 g, 5.3 mmol), potassium carbonate (1.5 g, 11 mmol) and acetonitrile (20 mL) was refluxed for 24 h. The reaction mixture was extracted with ethyl acetate, washed with water and dried with MgSO₄. After removing the solvent under reduced pressure, 3a was obtained as colourless liquid without further purification. Yield: 93%. ¹H NMR (400 MHz, CDCl₃) δ 7.39 (d, J = 8.6 Hz, Ar–H, 2H), 7.17 (d, J = 8.6 Hz, Ar–H, 2H), 2.89 (t, J = 7.4 Hz, Ar–S–CH₂, 2H) 1.63 (tt, J = 7.4 and 7.5 Hz, S–CH₂–CH₂, 2H), 1.41 (tt, J = 7.0 and 7.5 Hz, S–CH₂–CH₂–CH₂, 2H), 1.33–1.25 (m, CH₂–CH₂–CH₃, 4H), 0.88 (t, J = 7.0 Hz, CH₃, 3H) ppm.

1-Hexylsulfanyl-4-[2-(trimethylsilyl)ethynyl]benzene (3b). In a two-way flask, a mixture of 3a (1.4 g, 4.9 mmol), trimethylsilylacetylene (0.82 mL, 5.9 mmol), TEA (10 mL) and THF (10 mL) was bubbled with argon. Pd(PPh₃)₄ (0.11 g, 99 μmol) and CuI (19 mg, 99 μmol) were put into another two-way flask and it was filled with argon, and then former liquid mixture was added into it, and the mixture was stirred at 60 °C.

After 12 h, insoluble solid was filtrated and the mixture was extracted with ethyl acetate, washed with 2 M HCl aq and brine and dried with MgSO₄. After removing the solvent under reduced pressure, the residue was purified with silica gel on column chromatography (eluent: hexane) to afford 3b as colourless liquid. Yield: 79%. ¹H NMR (400 MHz, CDCl₃) δ 7.35 (d, J = 8.3 Hz, Ar–H, 2H), 7.20 (d, J = 8.3 Hz, Ar–H, 2H), 2.92 (t, J = 7.4 Hz, S–CH₂, 2H) 1.64 (tt, J = 7.4 and 7.5 Hz, S–CH₂–CH₂, 2H), 1.42 (tt, J = 7.1 and 7.5 Hz, S–CH₂–CH₂–CH₂, 2H), 1.33–1.25 (m, CH₂–CH₂–CH₃, 4H), 0.88 (t, J = 7.1 Hz, CH₂–CH₃, 3H), 0.24 (s, Si–CH₃, 9H) ppm.

1-Ethynyl-4-hexylsulfanylbenzene (3c)^4h,i. A mixture of 3b (1.1 g, 3.9 mmol), potassium carbonate (1.6 g, 12 mmol), MeOH (10 mL) and THF (10 mL) was stirred for 3 h, and the reaction mixture was extracted with ethyl acetate, washed with water and brine, and the combined organic layers were dried with MgSO₄. After removing the solvent under reduced pressure, 3c was obtained as colourless liquid without further purification. Yield: 99%. ¹H NMR (400 MHz, CDCl₃) δ 7.38 (d, J = 8.3 Hz, Ar–H, 2H), 7.22 (d, J = 8.3 Hz, Ar–H, 2H), 3.07 (s, –C≡CH, 1H), 2.93 (t, J = 7.3 Hz, Ar–S–CH₂, 2H), 1.65 (tt, J = 7.3 and 7.6 Hz, S–CH₂–CH₂, 2H), 1.43 (tt, J = 6.9 and 7.6 Hz, S–CH₂–CH₂–CH₂, 2H), 1.34–1.22 (m, CH₂–CH₂–CH₃, 4H), 0.89 (t, J = 7.1 Hz, CH₃, 3H) ppm.

4-[2-(4-Hexylsulfanylphenyl)ethynyl]benzoic acid (3). Compound 3 was synthesized according to the above procedure for Sonogashira coupling. The purification was performed by recrystallization with chloroform. 3c (0.85 g, 3.9 mmol), 4-bromobenzoic acid (0.78 g, 3.9 mmol), Pd(PPh₃)₄ (0.22 g, 0.19 mmol), CuI (36 mg, 0.19 mmol), THF (10 mL) and TEA (10 mL) were used. Colourless solid. Yield: 39%. ¹H NMR (400 MHz, DMSO-d₆) δ 7.97 (d, J = 8.6 Hz, Ar–H, 2H), 7.65 (d, J = 8.6 Hz, Ar–H, 2H), 7.51 (d, J = 8.5 Hz, Ar–H, 2H), 7.34 (d, J = 8.5 Hz, Ar–H, 2H), 3.02 (t, J = 7.3 Hz, Ar–S–CH₂, 2H), 1.60 (tt, J = 7.3 and 7.4 Hz, S–CH₂–CH₂, 2H), 1.40 (tt, J = 7.1 and 7.4 Hz, S–CH₂–CH₂–CH₂, 2H), 1.31–1.23 (m, CH₂–CH₂–CH₃, 4H), 0.86 (t, J = 7.1 Hz, CH₃, 3H) ppm. ¹³C NMR (100 MHz, DMSO-d₆) δ 167.6, 140.0, 132.5, 131.9, 131.0, 130.1, 127.5, 127.2, 119.0, 92.8, 89.7, 32.1, 31.7, 29.2, 28.8, 22.9, 14.8 ppm. FT-IR 3200–2500, 2956, 2925, 2853, 2213, 1681, 1601, 1425, 950 cm⁻¹. HRMS-FAB+ (m/z): [M] calcd for C₂₁H₂₂O₂S, 338.1341; found, 338.1331. Anal. calcd for C₂₁H₂₂O₂S: C 74.52, H 6.55, S 9.47%; found C 74.80, H 6.65, S 9.14%.

Results and discussion

Molecular design

LC materials based on diphenylacetylene are very important, not only for basic LC chemistry but also for various optical applications including liquid crystalline displays (LCDs).¹³ To explore the effects of sulfur in such materials, we synthesized a variety of sulfur-containing tolane-based compounds, as shown in Chart 1, and investigated their thermal transition behaviours. A symmetrical analogue with hexylsulfanyl groups at the p- and p′-positions (T1: –SC₆H₁₃) was prepared, and for comparison, analogues with hexyl (T2: –C₆H₁₃) and hexyloxy (T3: –OC₆H₁₃) groups were designed to investigate the effects of different tails. The non-substituted analogue (T4: –H) and that with a cyano group (T5: –CN) were prepared to investigate the effects of asymmetry; the latter analogue, especially, is known to form an antiparallel alignment between adjacent molecules in the mesophase.¹⁴ The thermal transition behaviours of these compounds were investigated by POM and DSC, and their transition temperatures are shown in Fig. 1. Even though tolane-based materials are known as LC materials with well-defined mesophases, not all of the sulfur-containing compounds exhibited enantiotropic mesophases. Only the hexyl-substituted analogue (T2) showed a monotropic smectic phase during the cooling scan in the POM and DSC measurements, but the LC range was just 13 °C, indicating mesophase instability. We also synthesized a diphenyldiacetylene (DPDA)-type molecule with two hexylsulfanyl groups (DPDA-SC6). This skeleton has a higher aspect ratio than that of tolane, and it is known that such compounds tend to form wide nematic phases owing to their undulating alignment in the mesophases.^7,15 However, no mesophases were observed, and only melting and crystallization points were detected for DPDA-SC6, even though it has enough anisotropic structure to generate mesophases.

Fig. 1 Transition temperatures of various sulfur-containing tolane-based and diphenyldiacetylene molecules.

We attempted to exploit the hydrogen-bonding properties of carboxylic acids to generate stable mesophases, because we conjectured that the disorder and/or flexibility induced by dimerization through hydrogen bonding would allow these compounds to translate and/or rotate to generate mesophases.

Therefore, we designed tolane-based molecules with a hexylsulfanyl group and a carboxylic acid at the p- and p′-positions, respectively. For comparison, analogues with alkyl and alkoxy groups were also synthesized, as shown in Chart 1. A representative synthesis is shown in Scheme 1 for compound 3; the syntheses of compounds 1 and 2 are described in the ESI.† Finally, as can be seen in the representative DSC curve of compound 3 (Fig. 2), we succeeded in generating stable enantiotropic mesophases for a thioether-containing tolane-based molecule, as well as compounds 1 and 2. Clearly, these mesophases are induced by hydrogen bonding, because other crystal tolanes and diphenyldiacetylenes with alkylsulfanyl groups possess enough anisotropy in their structure to exhibit mesophases. Structural analyses of all materials were performed using ¹H NMR, ¹³C NMR, high-resolution mass spectrometry (HRMS), and FT-IR spectroscopy (see ESI†). The broad O–H stretching at 2500–3200 cm⁻¹ in the FT-IR spectra for compounds 1–3 confirms the formation of dimers in the crystal state (Fig. S1†).

Scheme 1 Synthesis of compound 3.

Fig. 2 DSC curve of compound 3.

POM observations

The phase-transition behaviour of each compound was evaluated by DSC and POM. The transition temperatures and enthalpy changes detected by DSC are listed in Table 1. The DSC curves for compounds 1 and 2 are shown in Fig. S2 and S3,† respectively. As representative textures, the POM images of compounds 2 and 3 obtained using glass plates coated with a planar alignment polyimide, are shown in Fig. 3.

Table 1 Transition temperatures (°C) and enthalpy changes (kJ mol⁻¹)

Compound	Transition temperature/°C [enthalpy/kJ mol^−1]									Tm^a
a Measured on heating scan.b Highly ordered smectic phase.
1	Cr1	205.8 [3.2]	Cr2	213.6 [15.6]	CybC	219.3 [0.6]	N	255.8 (5.9) [5.9]	I	260.2 [5.5]
2	Cr	212.3 [19.3]			CybC	224.8 [0.5]	N	259.2 [6.3]	I	259.7 [5.9]
3	Cr	182.4 [13.1]	SmX^b	188.3 [1.8]	SmC	222.3 [0.9]	N	240.7 [5.1]	I	245.5 [5.5]

---

Fig. 3 POM images from in-plane alignment cells. (a) N phase at 230 °C and (b) CybC phase at 220 °C for compound 2, (c) N phase at 200 °C, and (d) SmC phase at 210 °C for compound 3.

For compound 2, a typical nematic (N) phase with marble and schlieren textures is observed on cooling from the isotropic (I) phase at 230 °C (Fig. 3(a)). Upon further cooling to 220 °C, a large number of defect lines appears over the marble texture with no other observable changes, such as the fan-like or broken fan-shaped textural transformations expected for the smectic phase (Fig. 3(b)). This behaviour is also observed for compound 1 (see Fig. S4†). The DSC measurements confirm that these transitions are first-order. These phases are assigned as cybotactic C (CybC) rather than general smectic C (SmC) phases, because they consist of short-range multiple domains of expanded cybotactic C clusters, as determined by XRD measurements (vide infra).

Compound 3 also exhibits a marble texture reminiscent of a typical nematic phase in its high-temperature liquid crystal state (Fig. 3(c)), which is enantiotropic. When cooled to the lower mesophase from the N phase, compound 3 notably shows a general ordered SmC phase with a broken fan-shaped texture (Fig. 3(d)). This result suggests that significant long-range correlation was achieved in this material as a result of the presence of the sulfur atom.

XRD measurements with non-aligned specimens

XRD measurements were performed to reveal the detailed phase structures of the compounds. One- and two-dimensional XRD (1D- and 2D-XRD) patterns of the compounds, obtained using non-aligned specimens, are shown in Fig. 4 and S5–S7,† respectively. From the 1D-XRD patterns of the N phases (Fig. 4(a)), the mean adjacent molecular distances, as determined from the outer broad refractions, are 4.9 Å in compounds 1 and 2, and 4.7 Å in compound 3. The distance for the alkylsulfanyl derivative is the smallest because of the large dipole interactions and S–S contacts acting between adjacent molecules. Furthermore, the formation of dimers can be confirmed from the estimated d-spacings of compounds 1, 2, and 3 corresponding to the small-angle region (27.4, 33.2, and 41.4 Å, respectively), and their calculated molecular lengths (18.0, 19.7, and 20.4 Å, respectively). These results indicate that the mesophases for these molecules are induced by the introduction of the hydrogen-bonding carboxylic acid unit.

Fig. 4 (a) 1D-XRD patterns, normalised by wide-angle intensity, of the N phase of compound 1 at 240 °C (dashed line), compound 2 at 254 °C (dotted line), and compound 3 at 230 °C (solid line). (b) 1D-XRD patterns of the CybC phase of compound 1 at 218 °C (dashed line), compound 2 at 222 °C (dotted line), and the SmC phase of compound 3 at 210 °C (solid line).

As shown in Fig. 4(a), the diffraction intensities in the small-angle region are larger than those in the wide-angle region for all the compounds, indicating the formation of cybotactic clusters in the N phases.¹⁶ The intensity ratios of the small- and wide-angle peaks (I_sax/I_wax) were measured and found to be 1.0 for compound 1 at 240 °C, 1.5 for compound 2 at 254 °C, and 2.4 for compound 3 at 230 °C. It is noteworthy that the alkylsulfanyl derivative exhibits a high ratio, over 2.0, suggesting that it forms larger clusters in the fluid N phase compared with the alkyl and alkoxy derivatives.

The small-angle diffractions of the low-temperature mesophase under the N phases of compounds 1 and 2 are wider than those of typical smectic phases (Fig. 4(b)), suggesting that the domains of compound 1 and 2 have short-range correlations and are dynamic, as the distributions are not Gaussian. The I_sax/I_wax ratio is 1.4 for compound 1 at 218 °C and 1.5 for compound 2 at 223 °C; these values are not much higher than those for the corresponding N phases. Hence, these phases are assigned as CybC phases consisting of multiple cybotactic C cluster domains, as reported for bent-type LC molecules.¹⁷ Furthermore, there are no striking differences between the 2D-XRD patterns of the CybC phases for compounds 1 and 2 (see Fig. S5 and S6†). Thus, these compounds exhibit disorder induced by hydrogen bonding, and their highly anisotropic molecular structures form dimers, which could not have been formed by their domains in normal smectic layers. In contrast, the diffraction peak of compound 3 is remarkably sharp (Fig. 4(b)), i.e. it corresponds to a conventional smectic phase. At 210 °C, (001) and (002) diffraction peaks are observed at diffraction angles of 2.5 and 5.0 with a 1 : 2 intensity ratio, corresponding to d-spacings of 35.4 and 17.7 Å, respectively. From these results, it is evident that the introduction of the sulfur atom greatly enhances the correlation length in the entire LC phase, including not only the fluid N phase but also the higher-order Sm phase, because its van der Waals attractions are larger than those of carbon and oxygen atoms. Another monotropic transition was detected in the POM images and DSC results under the first typical SmC phase. This transition is characterized as a highly ordered tilted Sm phase (SmX). However, we did not focus on this phase in the current study.

XRD measurements using the aligned specimens

XRD measurements were also conducted for specimens oriented by applying a magnetic field to elucidate the phase structures in detail. The 2D-XRD patterns obtained for compounds 2 and 3 are shown in Fig. 5, and the temperature-dependence of the 1D-XRD pattern for each compound is shown in Fig. S8–S10.† The molecular long axis is oriented along the longitudinal direction, as the diffractions in the wide-angle region are along the direction of the magnetic field, which is represented by an arrow. Split diffractions can be observed for all compounds in the small-angle region for the N phase (Fig. 5(a) and (b)), indicating the presence of cybotactic C clusters.¹⁸ The splitting maxima of the small-angle scatterings (χ/2) are estimated to be 54° at 230 °C for compound 2 and 45° at 225 °C for compound 1. It is noteworthy that the χ/2 value for compound 3 is 26° at 224 °C, which is approximately half that of the other compounds (Fig. 5(c)). This result can be attributed more to the S–S contacts between sulfur atoms in neighbouring molecules than their dipole interactions, because of the reduction in the angle caused by their proximity.

Fig. 5 2D-XRD patterns measured for aligned specimens. (a) N phase at 230 °C for compound 2, (b) N phase at 224 °C for compound 3, and (c) SmC phase at 216 °C for compound 3.

We further investigated the dependence of the phase structures of compound 3 on temperature. The obtained diffraction patterns are shown in Fig. S10.† The SmC phase shows a tilt angle of 27° at 212 °C. The angle for the SmC phase is similar to the χ/2 value for the N phase, indicating that, in this case, the cybotactic nematic phase corresponds to a pre-transition state to the smectic C phase. When the temperature of the SmC phase is decreased to 194 °C, the tilt angle from the normal layer is approximately 28°.

From these mesophase structure analyses, we presume that not only the highly anisotropic molecular structures, but also the disorder and/or flexibility arising from hydrogen bonding, allows these crystal molecules to rotate or translate to generate the LC phases. Even the monomeric structures of tolane- and DPDA-based molecules have enough anisotropic structure to generate mesophases. We also confirmed that esterified compound 3 (T6) did not display any mesophases; only a melting point of 108 °C was detected (Chart 1). This result emphasizes the importance of the hydrogen-bonding effect in the generation of mesophases for sulfur-containing rod-like molecules. Compounds 1 and 2 exhibit uncommon disordered phases, in contrast to compound 3, which has long-range correlated mesophases due to its alkylsulfanyl groups. In addition, several reports have indicated that the introduction of hydrogen bonds into a mesogenic structure increases the degree of disorder or flexibility, in contrast to what is seen for LC compounds with mesogens linked by covalent bonds.¹⁹

Birefringence measurements

As part of an effort to develop highly birefringent materials and to evaluate the effects of sulfur on the optical properties of these compounds, we measured birefringence (Δn) values in the N phase using microscopic spectroscopy.⁸ We have previously described this method in detail.⁹ The typical transmittance data and their fittings, as well as the birefringence of compound 3 at 550 nm and reduced temperatures (T_i − T), are shown in Fig. 6. As can be seen, the Δn values of the compounds can be arranged in the following order: 3 > 2 > 1. In particular, the maximum value obtained for Δn is 0.36 at 227 °C for compound 3. It is noteworthy that the temperature dependence of the birefringence is the greatest for compound 3. The temperature dependence of Δn is given by Δn = Δn_o(1 − T/T_i)^β,²⁰ where Δn_o is the birefringence for a perfectly oriented material (order parameter; s = 1) and β is a constant that is characteristic of the nematic material. The Δn_o values approximated by fitting to the above equation reach the extremely high value of 0.80 (β = 0.23) for compound 3, whereas those of compounds 1 and 2 are 0.49 and 0.50, respectively. These results are likely due to the greater polarisability and stronger attractive interactions of the sulfur atoms. Investigations on the dependence of the liquid crystallinity of an analogue on alkyl carbon number are in progress.

Fig. 6 Birefringence on their N phases of each compound at reduced temperature (T_i − T).

Conclusion

We designed, synthesized, and characterized a hydrogen-bonded tolane-based LC molecule with terminal carboxyl and alkylsulfanyl groups, and compared it to its alkyl and alkoxy analogues. All three derivatives successfully exhibited enantiotropic mesophases. We concluded that these mesophases were induced by the flexibility and/or disorder derived from the hydrogen bonding of the carboxyl groups, because non-hydrogen bonded rod-like molecules with alkylsulfanyl groups, including tolanes and diphenyldiacetylenes, which possessed sufficient anisotropy in their molecular structures to generate mesophases, did not do so. On analysing the mesophase structures of the prepared compounds, the alkylsulfanyl derivative was found to exhibit larger numbers of highly correlated mesophases than the alkyl and alkoxy derivatives in not only the fluid N phases but also the higher order Sm phases. This is attributed to the larger van der Waals attractions, including dipole interactions and S–S contacts, of the sulfur atoms. In particular, the alkyl and alkoxy derivatives formed highly disordered cybotactic C phases, whereas the alkylsulfanyl derivative exhibited an ordered smectic C phase. In addition, the birefringence of the alkylsulfanyl derivative was enhanced more than those of the alkyl and alkoxy analogues, because of the higher polarisability of the sulfur atom. Moreover, the birefringence was greatly influenced by temperature, with a maximum value of 0.36. We anticipate that comparable behaviour will be observed in other rod-like molecules with hydrogen bonding capability, which would help improve the optical and electronic properties of LC materials.

Notes and references

1. (a) T. M. Barclay, A. W. Cordes, R. C. Haddon, M. E. Itkis, R. T. Oakley, R. W. Reed and H. Zhang, J. Am. Chem. Soc., 1999, 121, 969; (b) K. Kobayashi, H. Masu, A. Shuto and K. Yamaguchi, Chem. Mater., 2005, 17, 6666; (c) K. Kobayashi, R. Shimaoka, M. Kawahata, M. Yamanaka and K. Yamaguchi, Org. Lett., 2006, 8, 2385; (d) Y. S. Yang, T. Yasuda, H. Kakizoe, H. Mieno, H. Kino, Y. Tateyama and C. Adachi, Chem. Commun., 2013, 49, 6483; (e) T. Okamoto, T. Suzuki, H. Tanaka, D. Hashizume and Y. Matsuo, Chem.–Asian J., 2012, 7, 105; (f) L. Tan, Y. Guo, G. Zhang, Y. Yang, D. Zhang, G. Yu, W. Xu and Y. Liu, J. Mater. Chem., 2011, 21, 18042; (g) S. Zou, Y. Wang, J. Gao, X. Liu, W. Hao, H. Zhang, H. Zhang, H. Xie, C. Yang, H. Li and W. Hu, J. Mater. Chem. C, 2014, 2, 10011; (h) Q. Xiao, T. Sakurai, T. Fukino, K. Akaike, Y. Honsho, A. Saeki, S. Seki, K. Kato, M. Takata and T. Aida, J. Am. Chem. Soc., 2013, 135, 18268.
2. (a) Y. Mazaki and K. Kobayashi, J. Chem. Soc., Perkin Trans. 2, 1992, 761; (b) R. S. Rowland and R. Taylor, J. Phys. Chem., 1996, 100, 7384; (c) J. Dai, M. Munakata, L.-P. Wu, T. Kuroda-Sowa and Y. Suenaga, Inorg. Chim. Acta, 1997, 258, 65; (d) M. Iwaoka, S. Takemoto, M. Okada and S. Tomoda, Bull. Chem. Soc. Jpn., 2002, 75, 1611; (e) Z. García-Hernández, A. Flores-Parra, J. M. Grevy, Á. Ramos-Organillo and R. Contreras, Polyhedron, 2006, 25, 1662; (f) D. B. Werz, R. Gleiter and F. Rominger, J. Am. Chem. Soc., 2002, 124, 10638; (g) R. Gleiter and D. B. Werz, Chem. Lett., 2005, 34, 126; (h) G. Mehta, V. Gagliardini, C. Schaefer and R. Gleiter, Org. Lett., 2004, 6, 1617; (i) M. A. Stokes, R. Kortan, S. R. Amy, H. E. Katz, Y. J. Chabal, C. Kloc and T. Siegrist, J. Mater. Chem., 2007, 17, 3427.
3. (a) A. Seed, Chem. Soc. Rev., 2007, 36, 2046; (b) M. Funahashi and J. Hanna, Adv. Mater., 2005, 17, 594; (c) A. Matsui, M. Funahashi, T. Tsuji and T. Kato, Chem.–Eur. J., 2010, 16, 13465; (d) T. Yasuda, T. Shimizu, F. Liu, G. Ungar and T. Kato, J. Am. Chem. Soc., 2011, 133, 13437; (e) J. I. Tietz, J. R. Mastriana, P. Sampson and A. J. Seed, Liq. Cryst., 2012, 39, 515; (f) S. Kaur, L. Tian, H. Liu, C. Greco, A. Ferrarini, J. Seltmann, M. Lehmann and H. F. Gleeson, J. Mater. Chem. C, 2013, 1, 2416; (g) Y. Arakawa, S. Kang, S. Nakajima, K. Sakajiri, S. Kawauchi, J. Watanabe and G. Konishi, Liq. Cryst., 2014, 41, 642; (h) S. Kuiper, W. F. Jager, T. J. Dingemans and S. J. Picken, Liq. Cryst., 2009, 36, 389; (i) A. Kovářová, J. Svoboda, V. Novotná, M. Glogarová, M. Salamonczyk, D. Pociecha and E. Gorecka, Liq. Cryst., 2010, 37, 1501; (j) A. Kovářová, M. Kohout, J. Svoboda and V. Novotná, Liq. Cryst., 2014, 41, 1703; (k) J. Herman, O. Chojnowska, P. Harmata, R. W. Dąbrowski, B. Klus and P. Kula, Liq. Cryst., 2014, 41, 1647.
4. (a) F. Araoka, S. Masuko, A. Kogure, D. Miyajima, T. Aida and H. Takezoe, Adv. Mater., 2013, 25, 4014; (b) D. Adam, P. Schumacher, J. Simmerer, L. Haussling, K. Siemensmeyer, K. H. Etzbach, H. Ringsdorf and D. Haarer, Nature, 1994, 371, 141; (c) S. H. Kang, Y.-S. Kang, W.-C. Zin, G. Olbrechts, K. Wostyn, K. Clays, A. Persoons and K. Kim, Chem. Commun., 1999, 1661; (d) F. Nekelson, H. Monobe, M. Shiro and Y. Shimizu, J. Mater. Chem., 2007, 17, 2607; (e) S. K. Varshney, H. Nagayama, V. Prasad and H. Takezoe, Liq. Cryst., 2011, 38, 1321; (f) J. Cattle, P. Bao, J. P. Bramble, R. J. Bushby, S. D. Evans, J. E. Lydon and D. J. Tate, Adv. Funct. Mater., 2013, 23, 5997; (g) S. Kumar, Liq. Cryst., 2004, 31, 1037; (h) P. H. J. Kouwer, W. F. Jager, W. J. Mijs and S. J. Picken, J. Mater. Chem., 2003, 13, 458; (i) P. H. J. Kouwer, W. F. Jager, W. J. Mijs and S. J. Picken, Mol. Cryst. Liq. Cryst., 2004, 411, 1347.
5. (a) I. C. Pintre, J. L. Serrano, M. B. Ros, J. Ortega, I. Alonso, J. M. Perdiguero, C. L. Folcia, J. Etxebarria, F. Goc, D. B. Amabilino, J. P. Luis and E. G. Nadal, Chem. Commun., 2008, 2523; (b) S. Kang, M. Harada, X. Li, M. Tokita and J. Watanabe, Soft Matter, 2012, 8, 1916; (c) G. Heppke, D. D. Parghi and H. Sawade, Liq. Cryst., 2000, 27, 313.
6. (a) A. J. Seed, K. J. Toyne, J. W. Goodby and D. G. McDonnell, J. Mater. Chem., 1995, 5, 1; (b) A. J. Seed, K. J. Toyne and J. W. Goodby, J. Mater. Chem., 1995, 5, 2201; (c) A. J. Seed, K. J. Toyne, M. Hird and J. W. Goodby, Liq. Cryst., 2012, 39, 403; (d) G. J. Cross, A. J. Seed, K. J. Toyne, J. W. Goodby, M. Hird and M. C. Artal, J. Mater. Chem., 2000, 10, 1555.
7. (a) Y. Arakawa, S. Nakajima, S. Kang, G. Konishi and J. Watanabe, J. Mater. Chem., 2012, 22, 14346; (b) Y. Arakawa, S. Kang, S. Nakajima, K. Sakajiri, Y. Cho, S. Kawauchi, J. Watanabe and G. Konishi, J. Mater. Chem. C, 2013, 1, 8094.
8. (a) Y. Arakawa, S. Nakajima, S. Kang, M. Shigeta, G. Konishi and J. Watanabe, J. Mater. Chem., 2012, 22, 13908; (b) S. Kang, S. Nakajima, Y. Arakawa, M. Tokita, J. Watanabe and G. Konishi, Polym. Chem., 2014, 5, 2253; (c) Y. Arakawa, S. Nakajima, S. Kang, G. Konishi and J. Watanabe, Liq. Cryst., 2012, 39, 1063; (d) Y. Arakawa, S. Kang, J. Watanabe and G. Konishi, Chem. Lett., 2014, 43, 1858.
9. (a) Y. Arakawa, S. Nakajima, R. Ishige, M. Uchimura, S. Kang, G. Konishi and J. Watanabe, J. Mater. Chem., 2012, 22, 8394; (b) S. Kang, S. Nakajima, Y. Arakawa, G. Konishi and J. Watanabe, J. Mater. Chem. C, 2013, 1, 4222; (c) M. Uchimura, S. Kang, R. Ishige, J. Watanabe and G. Konishi, Chem. Lett., 2010, 39, 513.
10. (a) A. E. Bradfield and B. Jones, J. Chem. Soc., 1929, 2660; (b) G. W. Gray and B. Jones, J. Chem. Soc., 1953, 4179; (c) A. J. Herbert, Trans. Faraday Soc., 1967, 63, 555; (d) C. Kavitha, N. P. S. Prabu and M. L. N. M. Mohan, Mol. Cryst. Liq. Cryst., 2014, 592, 163; (e) M. Roohnikan, M. Ebrahimi, S. R. Ghaffarian and N. Tamaoki, Liq. Cryst., 2013, 40, 314; (f) T. Iwata, R. Miyata and A. Matsumoto, Chem. Lett., 2013, 42, 849.
11. (a) T. Kato and J. M. J. Frechet, J. Am. Chem. Soc., 1989, 111, 8533; (b) T. Kato, T. Uryu, F. Kaneuchi, C. Jin and J. M. J. Fréchet, Liq. Cryst., 1993, 14, 1311; (c) T. Kato, N. Mizushita and K. Kishimoto, Angew. Chem., Int. Ed., 2005, 45, 38.
12. (a) F. Vera, R. M. Tejedor, P. Romero, J. Barber, M. B. Ros, J. L. Serrano and T. Sierra, Angew. Chem., Int. Ed., 2007, 46, 1873; (b) K. Kishikawa, S. Nakahara, Y. Nishikawa, S. Kohmoto and M. Yamamoto, J. Am. Chem. Soc., 2005, 127, 2565; (c) J. Del Barrio, E. Blasco, C. Toprakcioglu, A. Koutsioubas, O. A. Scherman, L. Oriol and C. Sanchez-Somolinos, Macromolecules, 2014, 47, 897; (d) M. Amorín, A. Pérez, J. Barberá, H. L. Ozores, J. L. Serrano, J. R. Granja and T. Sierra, Chem. Commun., 2014, 688; (e) T. Kato and K. Tanabe, Chem. Lett., 2009, 38, 634; (f) A. G. Cook, A. Martinez-Felipe, N. J. Brooks, J. M. Seddon and C. T. Imrie, Liq. Cryst., 2013, 40, 1817; (g) D. Stewart and C. T. Imrie, J. Mater. Chem., 1995, 5, 223; (h) C. M. Paleos and D. Tsiourvas, Liq. Cryst., 2001, 28, 1127; (i) K. Kishikawa, M. Isaka, M. Takahashi, K. Saito and S. Kohmoto, Chem. Lett., 2011, 40, 1278; (j) M. D. Miranda, F. V. Chávez, T. M. R. Maria, M. E. S. Eusebio, P. J. Sebastião and M. R. Silva, Liq. Cryst., 2014, 41, 1743.
13. (a) G. W. Skelton, D. Dong, R. P. Tuffin and S. M. Kelly, J. Mater. Chem., 2003, 13, 450; (b) S. Gauza, H. Wang, C. H. Wen, S. T. Wu, A. J. Seed and R. Dabrowski, Jpn. J. Appl. Phys., 2003, 42, 3463; (c) S. Gauza, X. Zhu, W. Piecek, R. Dabrowski and S.-T. Wu, J. Disp. Technol., 2007, 3, 250; (d) Q. Song, S. Gauza, H. Xianyu, S. T. Wu, Y. M. Liao, C. Y. Chang and C. S. Hsu, Liq. Cryst., 2010, 37, 139; (e) O. Catanescua and L. C. Chien, Liq. Cryst., 2006, 33, 115; (f) J. Hermana, J. Dziaduszek, R. Dąbrowski, J. Kędzierski, K. Kowiorski, V. S. Dasari, S. Dhara and P. Kula, Liq. Cryst., 2013, 40, 1174; (g) J. Dziaduszek, P. Kula, R. Dąbrowski, W. Drzewiński, K. Garbat, S. Urban and S. Gauza, Liq. Cryst., 2014, 39, 239; (h) B. Li, W. He, L. Wang, X. Xiao and H. Yang, Soft Matter, 2013, 9, 1172; (i) S.-J. Wang, R.-Y. Zhao, S. Yang, Z.-Q. Yu and E.-Q. Chen, Chem. Commun., 2014, 8378; (j) E. Levert, S. Lacelle, E. Zysman-Colman and A. Soldera, J. Mol. Liq., 2013, 183, 59; (k) M. Hu, Z. An, J. Li, L. Mo, Z. Yang, J. Li, Z. Che and X. Yang, Liq. Cryst., 2014, 41, 1696; (l) D. V. Sai, P. Sathyanarayana, V. S. S. Sastry, J. Herman, P. Kula, R. Dabrowski and S. Dhara, Liq. Cryst., 2014, 41, 591.
14. (a) A. J. Leadbetter, R. M. Richardson and C. N. Colling, J. Phys., 1975, 3, C1–C37; (b) J. E. Lydon and C. J. Coakley, J. Phys., 1975, 3, C1–C45; (c) C. Amovilli, I. Cacelli, S. Campanile and G. Prampolini, J. Chem. Phys., 2002, 117, 15.
15. L. Zhu, H. Tran, F. L. Beyer, S. D. Walck, X. Li, H. Ågren, K. L. Killops and L. M. Campos, J. Am. Chem. Soc., 2014, 136, 13381.
16. A. de Vries, J. Mol. Liq., 1986, 31, 193.
17. C. Keith, A. Lehmann, U. Baumeister, M. Prehma and C. Tschierske, Soft Matter, 2010, 6, 1704.
18. A. De Vries, Mol. Cryst. Liq. Cryst., 1970, 10, 219.
19. (a) E. Barmatov, S. Grande, A. Filippov, M. Barmatova, F. Kremer and V. Shibaev, Macromol. Chem. Phys., 2000, 201, 2603; (b) Y. Uchida, K. Suzuki and R. Tamura, J. Phys. Chem. B, 2012, 116, 9791; (c) W. He, G. Pan, Z. Yang, D. Zhao, G. Niu, W. Huang, X. Yuan, J. Guo, H. Cao and H. Yang, Adv. Mater., 2009, 21, 2050.
20. I. Haller, Prog. Solid State Chem., 1975, 10, 103.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental section: synthesis and identification, POM imaging, DSC, XRD, and evaluation of birefringence. See DOI: 10.1039/c4ra15300f

---